Ionization Modes: EI

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    Features of EI


    1. EI is the most widely used ionization mode in GC/MS analysis. Almost all commercial GCMS instruments equip this mode as standard ionization.
    2. EI causes much fragmentation of a molecule whose spectral pattern is useful for identifying sample compound.
    3. Library search is available. EI mass spectra obtained by 70eV electron bombardment can be used for identification by comparing with spectra registered in the mass spectral library.
    4. An open type of ion source is used. Vacuum pressure inside the source, mainly determined by carrier gas, is about less than 10-2 Pa.

    Molecular Ion Produced by EI

    In electron ionization, electrons from a filament sometimes knock out an electron of molecules and ionize the same compounds without break-up. The produced ion has the same mass as the original compound except for a negligibly small mass of an electron. We call such an ion a "molecular ion". This ion is very important for determining the molecular weight.

    Molecular Ion Produced by EI

    Fragment Ion Produced by EI

    70eV-electrons, normally used in EI, have enough energy to break up compounds. We call this decomposition of a molecule "fragmentation". Generally speaking, the molecular ion M+* in a vibration state is first produced, then the fragment ion m+ is produced from this unstable molecular ion because of excess energy in M+*.

    Chemical bonds usually break at weak bonds. Analysis of fragment spectra provides us with information on the chemical structures of a compound.

    Fragment Ion Produced by EI

    Full View of EI Process

    It is said that either a stable molecular ion M+ or an unstable molecular ion M+* is produced by electron impact, the result depending on compounds. For example, straight-chain hydrocarbons with many carbons rarely show molecular peaks, especially for the compounds having many carbons, because the unstable molecular ion M+* easily decomposes into a fragment ion.

    For methane, the molecular ion is CH4+. If hydrogen is removed, the fragment ion CH3+ is formed. Formation probability of each ion is constant, so we can get a reproducible spectrum as long as the analytical conditions are the same.

    Producing EI Mass Spectrum

    This is a simulation for producing a mass spectrum. Ion production is a probability event. Nobody can predict on a head which ion will be created. As the number of ions increase, the spectrum approaches the library spectrum. The truly measured spectrum is obtained by accumulating ions for a given length of time. The longer the time, the better the spectrum.

    Producing EI Mass Spectrum

    Electron Energy and Mass Spectrum

    The figure shows a spectrum of ethylacetate (MW=88) for 70-eV electrons. Usually for GC/MS analysis, a 70eV electron is used for ionization. A reason to use this energy is that the mass spectra in many libraries, which are frequently used for identification of unknown sample peaks, are customarily measured with 70-eV electrons. When the electron energy is changed, the spectrum changes as per the following:

    Electron Energy and Mass Spectrum
    • Ion signal is most intense at around 70eV.
    • When the energy is decreased, the spectrum will be less fragmented and the signal intensity seriously decreases. Ions cannot be produced below about 10eV.

    LICK"14eV" ("70eV") 

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